Delegate Barbara J. Comstock’s congressional campaign attempted to stiff-arm reports Monday that she voted for Barack Obama in Virginia’s 2008 Democratic primary by claiming it was part of a conservative bid to affect the Democratic presidential primaries six years ago.

The campaign defended the vote for Mr. Obama by arguing that it was part of Operation Chaos, a movement spearheaded by conservative talk show host Rush Limbaugh in late February 2008 as Mr. Obama’s campaign was picking up steam and had won several consecutive primaries over Hillary Rodham Clinton.

The Fairfax Republican “had participated in Rush Limbaugh’s 2008 ‘Operation Chaos’ to keep the Democrat primary going and nominate who she thought was the weakest candidate who Republican John McCain could beat,” campaign manager Susan Falconer said in a statement.

The movement, however, was originally meant to ensure Mr. Obama got “bloodied up” politically, Mr. Limbaugh told Fox News in May 2008 as Mr. Obama was on the cusp of securing the nomination.

“He was riding the messianic trail,” Mr. Limbaugh said at the time. “The McCain campaign had sworn they were not going to criticize Obama. Who better than the Clintons to do it? But for that to happen … she needed to stay in.”

But pulling the lever for Mrs. Clinton would have been anathema to Mrs. Comstock, who had worked for years as a congressional staffer investigating Clinton-era scandals and then for Republican presidential contender Mitt Romney in 2008 and 2012. Mr. Romney had suspended his campaign before the 2008 Virginia presidential primaries on Feb. 12.

Mrs. Comstock, widely known as a formidable fundraiser, will square off against a handful of Republican candidates in an April 26 firehouse primary in the race to replace retiring Rep. Frank R. Wolf, for whom Mrs. Comstock once worked.

Under Virginia law, voters can cast a ballot in any primary they choose, but cannot vote in both a Democratic and Republican presidential primary in the same year.

Mrs. Comstock apparently opted for Mr. Obama on the assumption that the freshman senator would be a weaker candidate than the formidable Clinton machine to take on Mr. McCain, the presumptive Republican nominee.

Mrs. Comstock’s campaign dismissed the revelation as “old news,” saying that Republicans in her Northern Virginia House district had known about the vote since 2009 when she first ran for office and she personally told top-level Republicans, such as Mr. Wolf, about it.

“It was a nonissue for them then and they all strongly supported Barbara,” Mrs. Falconer said. “Barbara has won every election where she has been on the ballot because she’s been focused on a positive agenda, not attacking fellow Republicans.”

Regardless of the ends, most Republicans would not advertise such a vote.

Delegate Robert G. Marshall, Prince William Republican who is running against Mrs. Comstock, had a problem with what he described as a meddlesome quality to the general effort.

“I think it’s better to light the candle than curse the darkness,” Mr. Marshall said. “Someone who wants to be a leader needs to think these things out before they do it.”